The subject invention relates to a high speed system for reading data from remote tags and for writing data into those tags. There are a number of prior art object and vehicle identification systems which use electronic tags containing fixed data about the particular object. These electronic tags were originally developed by the Los Alamos National Laboratory at Los Alamos, New Mexico, for the Department of Agriculture to identify livestock animals. A tag containing the animal's identification data in coded, electronically readable form, is attached to or implanted in the animal. Electronic readers are placed at feeding stations to READ the tags as the animals passed nearby. These readers, also called interrogators, transmit a continuous RF signal which, in turn, is "backscatter-modulated" with the identification data from the tag. These tags have an antenna which reflects the modulated signal, containing the information from the tag, to the interrogator. These early systems are described in U.S. Pat. Nos. 4,075,632 and 4,360,810.
As the technology progressed, it was found that these systems for identifying remote moving objects had utility far beyond animal identification. For example, the tags may be attached to any moving or moveable objects, such as automobiles, shipping containers, or railroad cars. Alternatively, the interrogator may be attached to the moving vehicle and tags located in fixed positions along its path. The information in these tags can be used to keep track of the location and path of travel of these moving vehicles or other objects. Railroad rolling stock can be continually monitored at stations or along the tracks so that their location and identity can be continually tracked on a central computer. U.S. Pat. Nos. 4,739,328; 4,782,345; 4,786,907; 4,816,839; 4,835,377 and 4,853,705 all generally describe aspects of these systems, including antennas and programmers used in these identification systems.
In certain applications, it has become important not only to be able to READ the data contained in these electronic tags, but also to modify that data, or to WRITE data into the tags while either the tags or the interrogators, used for supplying the data to be written, are in motion. For example, it may be important not only to know the identity of a railroad car, but also its contents. The list of these contents may be compiled by a computer, and may then be written into a tag attached to a railroad car as it passes interrogators along its route. The system of this invention uses a very efficient coding technique which considerably lessens the communication time required between the tag and the interrogator while maintaining very high levels of reliability. This enables the tags or the interrogators to be moving at very high speeds.
For such high speed operation, the coding schemes of the prior art are not efficient enough. Data rates need to be increased from about 10 kilobaud to almost 200 kilobaud. At the same time, since conventional interrogators decode the tag information using limiting amplifiers, it is necessary to provide this more efficient code, yet still enable a tag to backscatter-modulate the data without introducing a transmitted d.c. signal component. These limiting amplifiers used in interrogators have trouble with signals having significant d.c. components.
One possible solution to the problem, which does not increase the d.c. component of the signal, is to increase the signal data rate by increasing the clock rate. The problem with this solution, however, is that higher clock rates require more current and thus more power, causing a concomitant reduction in tag battery life. Accordingly, even though some increase in clock rate may be necessary to achieve the required data rate, it became an objective of this invention to increase the code efficiency rather than to rely entirely on the more brute force approach of increasing clock rate.